Ethylene oxide (EO) is used as a chemical intermediate, primarily for the production of ethylene glycols (EG) but also for the production of ethoxylates, ethanol-amines, solvents and glycol ethers. It is produced by the direct oxidation of ethylene with oxygen or air. Ethylene and oxygen are passed over a silver catalyst, typically at pressures of 10-30 bar and temperatures of 200-300° C. The reaction is exothermic and a typical reactor consists of large bundles of several thousand tubes that are packed with catalyst. A coolant surrounds the reactor tubes, removing the reaction heat and permitting temperature control.
The product stream from the ethylene oxide reactor is supplied to an ethylene oxide absorber. The absorber has an initial quench section wherein the product stream is contacted with a cooled, recirculating aqueous quench stream and a basic solution is continuously added to the recirculating quench stream. Such a quench section is described in U.S. Pat. No. 4,822,926. The aqueous quench stream is said to neutralise acidic compounds such as acetic acid and formic acid that may have been formed in the reactor. Another quench section is described in U.S. Pat. No. 5,336,791.
The gas stream passes from the quench section to the main section of the ethylene oxide absorber where it is scrubbed with water to recover ethylene oxide. The resulting water stream, which is rich in ethylene oxide, is referred to as the fat absorbent and this is sent to an ethylene oxide stripper. In the ethylene oxide stripper, the ethylene oxide is stripped and a concentrated ethylene oxide stream is sent to ethylene oxide finishing processes such as condensation, distillation and re-absorption. The remaining liquids, referred to as the lean absorbent, are recycled to the ethylene oxide absorber.
High purity ethylene oxide can be chilled, stored and transported to customers. Alternatively, the ethylene oxide produced in the plant may be routed to an ethylene glycol unit. Ethylene glycol is typically manufactured by reacting ethylene oxide with an excess of water, typically at temperatures of 150-250° C. Under these conditions reaction rates are fast and no catalyst is required.
The reaction of ethylene oxide with water typically produces a glycol product stream consisting of almost 90 weight percent monoethylene glycol, the remainder being predominantly diethylene glycol, some triethylene glycol and a small amount of higher homologues. The glycol product stream is passed through successive distillation columns with decreasing pressure to remove water, which is returned to the ethylene glycol reactor. The mono-, di- and tri-ethylene glycol are separated by vacuum distillation.
Carbon steel has typically been used for reactor vessels and piping in ethylene oxide and ethylene glycol plants. In the “Ethylene Oxide” entry of Ullmann's Encyclopedia of Industrial Chemistry (1987 edition) it is stated that since ethylene oxide is noncorrosive, the reactors and the sections of the plant that convey ethylene oxide are usually made of mild steel. However, the present inventors have observed corrosion of sections of ethylene oxide/ethylene glycol (EO/EG) plants and have sought to understand the mechanism of this corrosion and thereby provide solutions for mitigating this corrosion.